The present invention relates generally to tone synthesizing circuitry, and more particularly to multi-frequency tone synthesizing circuitry that is particularly well adapted for synthesizing a multi-frequency busy signal.
Prior art circuitry for generating a multi-frequency busy signal typically includes two tone generators whose outputs are coupled to an analog summing amplifier. The fast busy tone or reorder tone that is utilized in present high-capacity radiotelephone systems is comprised of two tones, 770 Hz, which are interrupted at a 2 Hz rate. In radiotelephone systems, these same two tones are also utilized to generate other supervisory tones, such as an alert tone, a feedback tone and an intercept tone. Since two tone generators are required, these prior art busy signal generators are relatively expensive.
In the prior art radiotelephones described in U.S. Pat. Nos. 4,122,304 and 4,220,820, a busy signal is generated which consists of an 400 Hz tone interrupted at a 1.7 Hz rate. The circuitry of these radiotelephone must likewise include a second tone generator in order to generate a multi-frequency busy signal.
According to another prior art technique, a multi-frequency busy signal can be generated by storing in a memory digitzed samples coded according to conventional linear or non-linear coding formats, and reading out from the memory the digitized samples and decoding them to reconstruct a busy signal when desired. However, such circuitry is relatively expensive since a memory and a ditigal-to-analog converter is required.
Thus, none of the foregoing prior art busy signal generating circuitry provides an adequate multi-frequency busy signal with relatively simple and inexpensive circuitry.